An electric power steering apparatus reduces the steering force needed from a driver by drive-controlling an electric motor with a motor drive control part on the basis of a steering torque signal outputted by a steering torque detecting part and a vehicle speed signal outputted by a vehicle speed detecting part. Electric power steering apparatuses that use a brushless motor as the electric motor are known.
With an electric power steering apparatus that uses a brushless motor, because there is no dropping or fluctuation of motor output due to a voltage drop between brushes and a commutator, it is possible to obtain a stable auxiliary steering force. Because the inertia moment of the motor is small compared to that of a motor with brushes, a good steering feeling can be obtained at high straight-line speeds and when the steering wheel is turned back from one direction to the other.
However, when a brushless motor is used as the motor, instead of brushes and a commutator, it becomes necessary for the amount of motor current to be controlled in correspondence with the angle of the motor (the angular position of the rotor). For this, a motor angle detecting part for detecting the angle of the motor (the angular position of the rotor) and a motor current detecting part are provided, and PWM drive control of the brushless motor is carried out on the basis of output signals from the motor angle detecting part and the motor current detecting part.
The motor angle detecting part comprises for example a resolver and an RD (resolver digital) convertor part. Signals from the resolver are supplied continuously to the RD convertor part. The RD convertor part calculates the angle of the rotor with respect to the stator in the brushless motor (the rotor angular position) θ, and outputs a signal corresponding to this calculated angle θ.
The RD convertor part calculates an angular velocity ω of the rotor with respect to the stator in the brushless motor, and outputs a signal corresponding to the calculated angular velocity ω.
An apparatus that uses a resolver and an RD convertor part to detect the angle of the motor (the angular position of the rotor) like this is disclosed in Japanese Patent No. 3216491.
When an RD convertor part for obtaining the angular position of a rotor on the basis of output signals from a resolver like this is employed in an electric power steering apparatus, there has been the problem that the RD convertor part is expensive and raises the cost of the apparatus. Also, when the RD convertor is performing angle calculation with an extremely short period and for example spike noise intrudes, because the computation itself is reset (computation is interrupted) there is a possibility of the driver being subjected to an incongruous feeling.
Thus, an electric power steering apparatus has been awaited with which calculation of the motor angle (angular position of the rotor) from the outputs of a resolver is achieved with a device that is low-cost and does not easily suffer influences of noise.